In electrostatography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image) is formed on an insulative surface by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise radiation-induced discharge of a uniform potential previously formed on a surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of a pattern of electrostatic potential on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
One well-known type of electrographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectric continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged, with the toner particles acquiring a charge of one polarity and the carrier particles acquiring a charge of the opposite polarity. These opposite charges attract each other such that the toner particles cling to the surfaces of the carrier particles. When the developer is brought into contact with the electrostatic latent image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred to another surface, to which it then can be similarly fixed.
A number of requirements are implicit in such development schemes. Namely, the electrostatic attraction between the toner and carrier particles must be strong enough to keep the toner particles held to the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are thereby pulled away from the carrier particles and deposited in the desired amount on the latent image-bearing surface. In order to meet these requirements for proper development, the level of electrostatic charge on the toner and carrier particles should be maintained within an adequate range.
Many known dry, two-component electrostatographic developers contain thermoplastic toner particles and carrier particles that comprise a core material which may or may not be coated with a polymer. Such polymeric carrier coatings can serve a number of known purposes. One such purpose can be to aid the developer to meet the electrostatic force requirements mentioned above by shifting the carrier particles to a position in the triboelectric series different from that of the uncoated carrier core material in order to adjust the degree of triboelectric charging of both the carrier and toner particles. Another purpose can be to reduce the frictional characteristics of the carrier particles in order to improve developer flow properties. Still another purpose can be to reduce the surface hardness of the carrier particles so that they are less likely to break apart during use and less likely to abrade surfaces (e.g., photoconductive element surfaces) that they contact during use. Yet another purpose can be to reduce the tendency of toner material or other developer additives to become undesirably permanently adhered to carrier surfaces during developer use (often referred to as scumming). A further purpose can be to alter the electrical resistance of the carrier particles.
Many different types of polymers have been described as useful for one or more of these purposes, among which are, for example, various styrene and methacrylate polymers and copolymers thereof. For example, U.S. Pat. Nos. 4,209,550; 4,572,885; and 4,822,708, and British published Patent Specification 1,385,231, suggest that, among many other polymers, poly(styrene), poly(methyl methacrylate), and poly(styrene-co-methyl methacrylate) may serve one or more of these purposes.
However, while such carrier coatings can serve the above-noted purposes well, in some cases they do not adequately serve some or all of these purposes simultaneously. For example, in some developer compositions, styrene and methacrylate polymer carrier coatings can serve many of the above-noted purposes well, but, depending upon the nature of the toner particles, such carrier coatings can cause the toner to acquire a triboelectric charge that is too low for optimum developer performance. This is true in some negatively charged developers (developers in which the toner particles triboelectrically acquire a negative charge, and the coated carrier particles acquire a positive charge). This is particularly true in such developers where the carrier coating material comprises poly(methyl methacrylate) or a copolymer of p-t-butylstyrene and a C.sub.1 -C.sub.4 alkyl methacrylate and the toner particles comprise a polymeric binder which is a polyester having a glass transition temperature of approximately 500.degree. to 100.degree. C. and a weight average molecular weight of approximately 20,000 to 100,000. Such polyester toner binders are very useful and are widely used in producing electrophotographic color images because of their ability to minimize offset which occurs when some of the toner adheres to the paper, while some remains on the fusing roller and then bonds to the next piece of paper. The low toner charge is due to the fact that these polymeric carrier coating materials are not triboelectrically potent or efficient enough to achieve the desired degree of charging. Consequently, the toner particles are undercharged resulting in less than optimum developer performance and hence less than optimum image development and image quality.
Thus, there remains a need to establish and maintain a proper level of charge on the toner particles described above so that the desired level of charge on the toner particles required for optimum image development and image quality can be achieved. The present invention meets that need.